2.80 install wiped out 2.79
Download the archive version of 2.79b for your platform, which doesn't require installation.


Windows 64-bit
Windows 32-bit
Linux 64-bit
Linux 32-bit
macOS


Once extracted you should be able to run Blender 2.79b normally. Unlike the installer the archive doesn't give you a start menu entry or shortcut. You will need to create that shortcut yourself or start it directly from the directory the binary is in.